Pincer ligands are tridentate ligands that bind tightly to three adjacent coplanar sites of a metal center in a meridional fashion, which can be divided into different type according to the coordinate atoms, such as NCN-, PCP-type pincer ligands. The concept was first proposed in the 1970s. The corresponding metal complexes containing pincer ligands are named as pincer complexes. Pincer complexes are widely used in the field of coordination chemistry, organic synthesis, homogeneous catalysis and material chemistry due to their properties of excellent stability, reactivity and stereoselectivity (Angew. Chem. Int. Ed. 2001, 40, 3750-3781). In recent years, pincer complexes not only play an important role in fields of CC coupling reaction, inert chemical activation, but also have important applications in the field of solar cells, for example, the terpyridine ruthenium complex (black dye) being widely used as optoelectronic materials.
The structure, reactivity, as well as applications studies of pincer complexes have achieved significant progress since the pincer complex first reported in 1976 (J. Chem. Soc. Dalton Trans 1976, 1020-1024). At present, most of the reported pincer complexes contain pincer ligands of NCN- (Coord. Chem. Rev., 2007, 251, 610-641; Coord. Chem. Rev., 2004, 248, 2275-2282), NNN-, PCP type (Chem. Rev., 2003, 103, 1759-1792), PCO- or SCS-type, in which there is at least one hetero coordination atoms. Pincer complexes with carbon exclusively as the bonding atoms (i.e., CCC-type pincer complexes) are rare. In 2013, Xia et al. reported novel fused-ring metallacyclic compounds, metallapentalynes, which can be regarded as a new kind of CCC-type pincer complexes (Nat. Chem. 2013, 5, 698-703). These CCC-type pincer complexes exhibit unique properties, such as aggregation induced emission enhancement, large Stokes shifts and long lifetime, broad absorption from the ultraviolet-visible to the near-infrared region and excellent photoacoustic and photothermal properties, thus possess potential applications in biomedicine and solar energy utilization
Although CCC-type pincer complexes have a good application prospect, how to synthesize these complexes efficiently remains a big challenge due to lack of appropriate CCC pincer ligand or ligand precursor. Therefore, a development and synthesis of the CCC-type pincer ligand or CCC-type pincer ligand precursor are particularly important.